The invention relates to an electrical machine.
Magnetic bearings are being increasingly used in electrical machines because no friction losses occur on them, as in the case of mechanical bearings. However, magnetic bearings have the disadvantage that disturbances can lead to their failure. (The simplest disturbance situation is a conventional electrical power failure). If the disturbance occurs during operation of the electrical machine, the shaft of the electrical machine is still rotating at high speed when the magnetic bearing fails. It is therefore necessary to provide so-called safety bearings, which hold the shaft in the event of failure of the magnetic bearings, and guide it at least while it is brought to rest, for example during transition from the state in which the shaft is moving to it being brought to rest.
A first approach in the prior art for refinement of the safety bearings was to use essentially conventional roller bearings. In this case, the shaft is guided in the roller bearing with play, in which case it is normally guided by the magnetic bearing, with the roller bearing acting as a safety bearing only when the magnetic bearing fails. Roller bearings are successful, particularly in the case of small electrical machines, in which the shaft is not heavy. Roller bearings have the advantage that the so-called “backward whirl” is suppressed, rotation of the entire shaft in the opposite rotation direction to the rotation applied to it, about its rotation axis.
Roller bearings are not suitable for holding large, heavy rotors: there is only a point or line contact between the roller bodies and the bearing rings, thus resulting in very high surface pressures. These high loads can damage the roller bearings and can block the bearing. In the case of large, heavy shafts, such as those which are used in turbocompressors for feeding gas, dry sliding bearings are nowadays used as safety bearings. Dry sliding bearings essentially comprise a ring which is mounted on the stator and in which the shaft can then rotate in the event of failing of the magnetic bearings. Since the risk of backward whirl rises as the coefficient of friction between the shaft and the safety bearing increases, the aim is to minimize the coefficient of friction by choice of suitable materials for this ring. By way of example, specific bronze alloys are used as friction coatings, but they are subject to severe wear.
The problem of wear in a dry sliding bearing is solved in U.S. Pat. No. 5,739,609 by splitting the ring in two: one ring element is attached to the stator, and a second ring element is mounted in the first ring element. In the event of wear on the inner surface, the second ring element can then be replaced. A sliding means is used in this case in order to make it easier to replace the second ring element. However, during operation the second ring element remains rigid with respect to the first ring element, which is attached to the stator, so that nothing is fundamentally changed with regard to the method of operation as a dry sliding bearing.